A 34-year-old woman presented to the out-patient clinic with angioedema and type II hereditary angioedema was confirmed immunologically. She also volunteered she had never had a menstrual period and physical examination identified several features of Turner's syndrome. A mosaic karyotype with XY and XO was found on chromosomal analysis and gonadectomy was performed in view of the high risk of gonadoblastoma. After commencing oestrogen at physiological replacement doses, the patient experienced a marked deterioration in both the severity and frequency of angioedema attacks. Coexistence of hereditary angioedema and Turner's syndrome has not previously been reported and this case highlights the detrimental C1 inhibitor level lowering effect of oestrogen in hereditary angioedema.